This invention pertains to earth boring apparatus and more particularly to a rotary drilling head.
In conventional drilling by the rotary method the drill bit cuttings and other detritus are returned to the surface by the drilling fluid which flows back up the annulus around the drill string from the bottom of the hole. At the surface the fluid flows laterally out of the side of a bell nipple or open topped tee which surmounts the stack of drilling equipment, e.g. well head, master valve, and blowout preventers at the top of the well casing. In some cases it is desirable to provide a seal between the upper end of the drilling stack and the rotating drill string extending therethrough. To effect such a seal, the bell nipple at the top of the stack is replaced by apparatus known variously as a drilling head, rotary drilling head, rotary blowout preventer, pressure drilling head, or diverter. Hereinafter such apparatus will be called a drilling head.
A drilling head may be employed with advantage in a variety of circumstances. For example, in gas drilling the drilling fluid may be natural gas, air, or other gas. A drilling head is needed for gas drilling fluid because the returning gas would otherwise tend to flow up toward the bottom of the rotary table and blow detritus onto the workmen on the rig floor whenever the master bushing or kelly bushing was removed from the rotary table.
In reverse circulation drilling, a drilling head is required in order that the annulus can be pressurized sufficiently to cause the drilling fluid (liquid or gas) to move down the annulus and back up the drill string.
In dual conduit drilling, a drilling head is desirable to seal the annulus to prevent drilling fluid from flowing up and out of the annulus.
In the case of geothermal drilling, a drilling head is desirable since the well bore will usually pass through many steam bearing formations. The steam in the drill string expands as it reaches the lower hydrostatic pressures in the top of the annulus and may emerge with the drilling fluid at high velocity.
Except for the seal, a drilling head is similar to a bell nipple in that it includes a tubular body having a side outlet and having a flange or other means at its bottom to support it on the blowout preventer or other drilling stack member and make a stationary seal therewith. In addition to all of the foregoing, a drilling head includes means to seal between the body of the head and the kelly or top joint forming the drive tube of the drill string.
Since the drive tube is normally rotating when the drilling head is in use, means to seal between a drilling head and drill string is quite different from the usual stationary packoff employed at the upper end of a tubing or casing head to seal between such production well head and a production string. A further distinction arises from the fact that a kelly is of non-circular external cross-section, as distinct from the circular outer periphery of tubing or casing pipe. Even, in the case where a power swivel is employed to rotate the drill string and the drive tube is a top joint of the drill string, the outer periphery of the top joint, though generally circular in cross-section, is apt to be rough, as distinct from a polished joint which may sometimes be employed at the upper end of a production string.
The head to string seal means of a drilling head is also to be distinguished from the seal means employed in a blowout preventer, for even in a drilling blowout preventer the seal means usually closes about a kelly or other drive tube that is stationary. However, a preventer must also close about a non-circular outer periphery member such as a kelly, or about a top joint which may be rough. It is perhaps for this reason that a drilling head is sometimes called a rotating blowout preventer.
Because of the problem of sealing with a rotating irregularly shaped drive tube, the head to string seal is divided into two parts. This is accomplished by dividing the drilling head itself into two parts, rotor and stator, the stator being a well head including a tubular body with a bottom flange and side outlet. The rotor is provided with means to make a non-rotating seal with the drive tube (kelly or top joint) and there is provided a rotating seal between circular cross-section surfaces of the rotor and stator. Since the rotor seals with both the drill string and the well head body forming the stator, it may be called a seal tube. The means to seal between the seal tube and well head body may be called a head seal. The means to seal between the seal tube and drill string may be called a string seal.
The string seal may be a large tubular mass of elastomeric material having an inner configuration similar to but smaller than that of the kelly with which it is to seal, i.e., a square or hexagonal or other interior configuration according to whether the kelly has a square or hexagonal or other configuration. Alternatively, the string seal may have a cross-section whose inner periphery is circular with a diameter smaller than that of the minimum transverse dimension of the drive tube (kelly or top joint) and soft enough to form a seal about the drive tube when the latter extends therethrough. The string seal must also have sufficient elastic deformation to allow it to stretch over tool joints or other drill string couplings when the drill string is elevated and the kelly or top joint is pulled out of the string seal, as when adding a length of pipe to the drill string.
Since the string seal fits tightly about the drive tube, the friction therebetween or the engagement of the complementary non-circular cross-sections thereof will cause the seal tube to be rotated with the drill string drive tube and there will be no relative rotational sliding therebetween, although there will be slow relative axial motion as drilling progresses and the drill string lowers into the well bore. To insure that there is no relative rotation between the seal tube and drive tube when the latter is a kelly, a metal drive bushing may be employed between the kelly and seal tube. The drive bushing is a tube having non-circular cross-section inner and outer peripheries, the inner periphery being correlative to the kelly and the outer periphery being correlative to an opening in the seal tube. The drive bushing is made in two parts, being divided axially, so that it can be placed about the kelly, and then bolted together. The drive bushing will be below the usual kelly bushing employed to engage the kelly with the rotary table. While the drive bushing is sometimes called a kelly bushing, such nomenclature creates a confusion with the regular kelly bushing for the rotary table; therefore the term drive bushing is employed hereinafter.
From the foregoing it will be seen that the rotor is a two part member including a seal tube and a drive bushing. Such construction is shown for example in U.S. Pat. Nos.:
2,243,439--Pranger (1941) PA1 2,303,090--Pranger (1942) PA1 2,222,082--Leman et al (1940) PA1 3,023,012--Wilde (1962). PA1 2,401,960--Pranger (1946) PA1 3,128,614--Auer (1964). PA1 APT Spec 7 PA1 Thirtieth Edition PA1 April 1975
The drive bushing will not pass over the connector at the lower end of the kelly. When it is necessary to elevate the kelly out of the drilling head, as when adding a joint of drill pipe or when removing the drill string from the well bore to change the drill bit, the drive bushing lifts out of the opening in the seal tube in which it is normally received. The kelly connector then passes up through the string seal and rotary table. If pipe is being pulled to change the bit, the string seal will also pass the tool joints connecting adjacent lengths of pipe.
However, the string seal will not pass drill string members of well bore diameter, such as reamers, stabilizers and drill bits. When such a member is encountered, or when periodically during the life of a drilling head it is necessary to replace the string seal and/or the head seal, the entire rotor may be lifted out of the stator, preferably leaving behind the anti-friction bearings that support the rotor in the stator. Such construction is shown in U.S. Pat. Nos.:
The removability of the rotor also makes it possible to remove the string seal at the beginning of a trip, if desired, thereby to avoid passing tool joints through the string seal and the consequent wear and tear.
Other patents showing both of the foregoing constructions are U.S. Pat. Nos.:
In the Pranger and the Auer constructions, the bearing means includes upper and lower axially separated cone or roller thrust bearings, to take both upward thrust due to annulus pressure and downward thrust due to weight of the rotor and frictional loads transferred from the kelly. With this arrangement, there apparently is insufficient space below the bearings to provide a side outlet connection flange inboard of the bearing, without making the drilling head unduly high. Pranger provides no side outlet at all, and Auer provides a protuberant side outlet. With a protuberant side outlet, the drilling head may be too large to pass through the opening in a rotary table that is left after the master bushing of such table has been removed, making it necessary to install the drilling head by moving it in laterally underneath the rotary table, working under the drill rig floor.
It is one object of the invention to overcome the foregoing difficulty and provide a drilling head incorporating all the desirable features such as a removable drive bushing, removable rotor, thrust bearing means for taking both upward and downward thrust, string seal and head seal means, and side outlet with connection means for making connection with a pipe flange, which will pass through the master bushing opening of a standard rotary drilling head. For standards one may refer to page 33 of:
an API Specification for Rotary Drilling Equipment, issued by the American Petroleum Institute, Division of Production, Dallas, Texas.
Other objects and advantages of the invention will appear from the following description thereof.